revert the cancellation sensitivity change for the flush task, doesn't work because flush task can't disambiguate orderly shutdown

pageserver/src/virtual_file/owned_buffers_io/write/flush.rs

@@ -20,11 +20,6 @@ pub struct FlushHandleInner<Buf, W> {
     /// A bi-directional channel that sends (buffer, offset) for writes,
     /// and receives recyled buffer.
     channel: duplex::mpsc::Duplex<FlushRequest<Buf>, FullSlice<Buf>>,
-    /// The flush task is sometimes sensitive to channel disconnection
-    /// (i.e. when we drop [`Self::channel`]), other times sensitive to
-    /// [`FlushBackgroundTask::cancel`], but never both.
-    /// So, also store this drop guard.
-    set_flush_task_cancelled: tokio_util::sync::DropGuard,
     /// Join handle for the background flush task.
     join_handle: tokio::task::JoinHandle<Result<Arc<W>, FlushTaskError>>,
 }
@@ -139,10 +134,8 @@ where
         back.try_send(buf.flush())
             .expect("we just created it with capacity 1");
 
-        let cancel = cancel.child_token();
-
         let join_handle = tokio::spawn(
-            FlushBackgroundTask::new(back, file, gate_guard, cancel.clone(), ctx)
+            FlushBackgroundTask::new(back, file, gate_guard, cancel, ctx)
                 .run()
                 .instrument(span),
         );
@@ -150,7 +143,6 @@ where
         FlushHandle {
             inner: Some(FlushHandleInner {
                 channel: front,
-                set_flush_task_cancelled: cancel.drop_guard(),
                 join_handle,
             }),
         }
@@ -197,7 +189,6 @@ where
             .take()
             .expect("must not use after we returned an error");
         drop(handle.channel.tx);
-        drop(handle.set_flush_task_cancelled);
         handle.join_handle.await.unwrap()
     }